The present invention relates to the preparation of a stable aqueous solution which comprises a polyene fungicide and the use of such a solution to treat food, feed, agricultural products, growth substrates of crops and other materials on which fungi, including moulds or yeasts, can grow. This solution can also be used for pharmaceutical or cosmetic purposes. Products, substrates and materials treated with the solution of the invention are also disclosed.
The prevention of fungal growth is an important topic throughout the world. Many food, feed, agricultural products or other natural products can be considered as good substrates for fungal growth. Also crops can easily suffer from fungal infection, especially when grown at high humidity or in poor weather conditions. It is also well known that the growth substrate of many crops can be a source of fungal contamination. Finally, fungi can also grow inside and outside buildings on e.g. building materials, equipment, walls and all kind of wooden materials. This can in some cases lead to severe fungal problems in houses and factories.
Apart from serious economic losses, fungal growth is also a severe health risk. Growth of fungi may lead to introduction of fungal mycotoxins into the food chain. Fungal growth in houses not only looks ugly, but also introduces fungal particles into the air which might lead to allergic reactions and breathing problems.
Polyene fungicides can be used to prevent fungal growth in a wide variety of applications. An example of a polyene fungicide is natamycin. For more than 30 years, natamycin has been used to prevent fungal growth on food products.
Treatment with natamycin is extremely effective in preventing fungal growth on solid products, which require protection for a longer period of time. Cheeses and sausages are well known examples of food products, where surface treatments with natamycin is used very successfully.
A natamycin suspension can be added to the polymer dispersion that is applied to the cheese rind as a coating. Whole cheeses, shredded cheese or sausages can also be treated with a suspension of natamycin in water by spraying. Finally cheeses and sausages can also be dipped in a natamycin suspension.
Such treatments are not only efficient but also very convenient for food products, which are ripened or stored in the open air. After the treatment the surface of the product, the surface contains a mixture of dissolved and solid natamycin. Only the dissolved fraction has antifungal activity, whereas the more stable crystals ensure a prolonged working time.
The level of dissolved natamycin continuously decreases due to its interaction with fungi, diffusion or physical and chemical degradation. However, the level of active dissolved natamycin is maintained for a long period of time due to the slow release of natamycin from the crystals and diffusion over the surface of the product. Under these practical conditions, the sensitivity of fungi is far below the solubility of natamycin and, as a result, this is a very effective method of preventing fungal growth. Under normal hygienic conditions, several applications of these kind of treatments have proven to be effective.
Natamycin has a minimal inhibitory concentration of less than 5 ppm for most yeasts and of less than 10 ppm for most foodborne fungi. The solubility of natamycin in water at neutral pH levels is 30 ppm (Brik, H.; “Natamycin” Analytical Profiles of Drug Substances 10, 513-561 (1981)). Therefore for many applications, a treatment with a natamycin suspension is quite effective: the level of dissolved active natamycin is high enough to prevent the outgrowth of fungi while the undissolved fraction forms a depot.
However, for many other applications, a prolonged working time is less (or not at all) required. For such applications, the undissolved inactive fraction can be considered as redundant. Moreover, during preparation, stable aqueous natamycin suspensions have hardly any antifungal activity available just after adding the natamycin powder to the water. Even in cases where stirring devices are used, it takes several hours before the maximum amount of 30 ppm natamycin is dissolved. In cases of high contamination levels, unhygienic conditions or less sensitive fungal species, even higher amounts of active natamycin than the actual solubility in water are desired. In all these cases, there is a need for a stable and ready-to-use formulation in which the optimal amount of active fungicide is available quickly.
The solubility of polyene fungicides can be improved using alkaline or acidic conditions. However, the fungicide dissolves rapidly and is also rapidly decomposed under such conditions. Therefore, in practice, such preparations will not be used.
Modified crystal structures of polyene fungicides, solvates such as methanol solvates and calcium or barium salts as described in European Patent Application No. 670676 as improving the release of the compound from the crystals resulting in an improved activity. These compounds have in general a higher potential of dissolving the polyene fungicide. However, the dissolved fraction will be transformed to the thermodynamically most stable solid form when suspended in an aqueous environment. The fast release is only temporary and will not actually enhance the solubility of the polyene fungicide.
It can be concluded that there is a need for a stable natamycin solution which a concentration above the maximum solubility in water wherein the natamycin can be dissolved rapidly. However, up to now, it has not been possible to prepare such stable solutions. Even the quick preparation of stable solutions within the known physical limitations was not possible.